The present invention generally relates to three-dimensional object imaging methods and systems, and more particularly to a three-dimensional object imaging method and a three-dimensional object imaging system which are applicable to automatic recognition systems for recognizing three dimensional objects, ultrasonic robot eye and the like.
There is active research in automatic recognition techniques for recognizing a three-dimensional shape using mainly a television camera for the purpose of improving factory automation. However, an automatic recognition system which satisfactorily processes three-dimensional information of an object has not yet been reduced to practice. The automatic recognition system which uses the television camera suffers from problems in that the quantity of input data is extremely large and it is impossible to process the three-dimensional information of objects such as a metal object having a mirror surface which reflects light and a transparent object through which light can transmit.
On the other hand, in order to realize an intelligent robot, it is desirable to develop a robot eye which can identify and measure objects and also recognize the outside. As conventional methods of recognizing the outside, various methods using light wave, X-ray, electromagnetic wave, sound wave and the like as the wave medium have been proposed, but no decisive method has emerged.
According to the method using the ultrasonic wave, it is possible to measure the general shape of the object. However, it is impossible to identify the object from the information obtained by the measurement or estimate the position and rotary angle of the object. For example, Watanabe et al., "The Ultrasonic Robot Eye System Using Neural Network", Electronic Information Communication Society, 2.22, 1989 proposes identification of a plannar object. However, when the object has a three-dimensional shape, the conventional method cannot identify the object, estimate the position and rotary angle of the object, or form an image of the object with a high definition.
The use of ultrasonic wave for recognition of the three-dimensional object has the following advantages.
First, it is possible to measure the phase of the scattering waves from the object and the change of the sound pressure with time because the propagation velocity of the sound wave is relatively slow, and as a result, it is possible to directly obtain three-dimensional information of the object. In the system which uses the light wave as the wave medium such as the system which uses the television camera, it is possible to obtain a two-dimensional image of the object relatively easily, however, it is difficult to accurately estimate information along the depth of the object since this requires a technique of analyzing the image. For this reason, even in the robot which uses the television camera as the main sensor, an ultrasonic sensor is provided in most cases for the purpose of detecting the existence of the object and measuring a distance between the robot and the object. As one application which utilizes the advantage of using the ultrasonic wave, there is a guidance system for the blind which is being developed.
Second, it becomes possible to recognize metal, transparent and black objects, fluids and the like by use of the ultrasonic wave. It is virtually impossible to recognize such objects by use of the television camera because of the large or small reflection, transparency and the like of these objects with respect to light. When the recognition system is applied to the factory automation, parts of the product may be made of a metal, and the method which uses the ultrasonic wave is thus better suited for recognizing such metal parts. Various systems which use the ultrasonic wave image for checking, recognition and the like have been reduced to practice. Examples of such systems are a system for automatically distinguishing 150 kinds of empty bottles in a supermarket, a system for automatically detecting a content level of a softdrink bottle, a system for automatically checking whether or not a straw is attached to the side of a paper container containing a softdrink and the like.
Third, by use of the ultrasonic wave, it becomes possible to make an object recognition even under difficult circumstances such as inside a turbid fluid, a dark room, a room filled with smoke and the like. To take advantages of this feature, there is research to use the ultrasonic robot eye for carrying out various tasks in deep water, for making rescue operations in fire and the like.
On the other hand, the object recognition method which uses the ultrasonic wave cannot match the high resolution obtainable on the television camera because the wavelength of the ultrasonic wave is long and there are limits to the size of the wave receiver array and the number of wave receivers. For this reason, the object recognition method which uses the ultrasonic wave is at the present confined to the use in making measurements on a living body and to the use underwater.
The so-called ultrasonic wave holography technique irradiates the ultrasonic wave on the object, measures space information of the reciprocal scattering wave from the object on a receiver transducer array, and calculates a reconstructed image of the object. However, this known technique generally introduces a large distortion in the reconstructed image and the resolution is too poor for practical use. These problems are caused mainly by the long wavelength of the ultrasonic wave, the limited size of the wave receiver array and the limited number of wave receivers in the wave receiver array.